This invention relates to frequency doublers.
In a television receiver, synchronization of the vertical scan to the horizontal scan is required for proper picture display. Lack of synchronization will cause picture roll and poor interlaced scanning. To provide for synchronization, many television systems make use of the horizontal flyback pulses derived from the horizontal deflection circuits. As described in U.S. Pat. No. 3,878,336 granted to A. R. Balaban, input signals occurring at the horizontal frequency rate f.sub.H obtained, for example, from the horizontal output transformer, are doubled in frequency. The frequency doubled output signals are then coupled to a vertical countdown system which drives the vertical deflection circuits. If the frequency doubler's output signals deviate from a 2f.sub.H rate even slightly, poor interlacing and a blurred picture result.
Some frequency doubling circuits use a 2f.sub.H vertical oscillator together with a phase detector, such as a phased-locked loop, to lock the oscillator to the f.sub.H input signals. A potentiometer is required for setting the oscillator frequency. Since many vertical countdown systems, such as described in the aforementioned patent, eliminate the vertical oscillator and its associated frequency set potentiometer, a frequency doubler which does not require any tuning elements or adjustment is desirable.